1. Field of the Invention
This invention relates to array antenna systems and particularly to such systems wherein the antenna element pattern is modified by providing a lossless spatial filter between the antenna input ports and the antenna elements so that the effective element pattern associated with each input port is primarily within a selected angular region of space.
2. Description of the Prior Art
An array antenna system may be designed to transmit a desired radiation pattern into one of a plurality of angular directions in a selected region of space. In accordance with the prior art designs of such array antennas, each of the antenna elements has an associated input port. By variation of the amplitude and/or phase of the wave energy signals supplied to the input ports, the antenna pattern can be electronically steered in space to point in a desired radiation direction or otherwise controlled to radiate a desired signal characteristic, such as a time reference beam scanning pattern. When it is desired to have an array antenna radiate its beam over a selected limited region of space, it is preferable that the radiation pattern of the individual antenna elements also be primarily within the selected angular region. This permits maximum element spacing while suppressing undesired grating lobes.
In certain systems, control of the element pattern by modification of the physical shape of the antenna element may be impractical because of a desired element pattern may require an element aperture size which exceeds the necessary element spacing in the array. A practical approach to overcome the physical elements size limitation is to provide networks for interconnecting each antenna input port with more than one antenna element, so that the effective element pattern associated with each input port is formed by the composite radiation of several elements. These networks can be realized by printed circuit techniques using a single substrate layer.
One prior art approach to this problem has been described by Nemit in U.S. Pat. No. 3,803,625, incorporated herein by reference. Nemit achieves a larger effective element size by providing intermediate antenna elements between the primary antenna elements and coupling signals from the primary antenna element ports to the intermediate element ports. This tapered multielement aperture excitation produces some measure of control over the radiated antenna pattern.
A more effective prior art antenna coupling network is described by Frazita et al. in U.S. Pat. No. 4,041,501 incorporated herein by reference and assigned to the same assignee as the present invention. According to the technique of Frazita, the antenna elements are arranged in element modules, each module is provided with an input port. Transmission lines are coupled to all of the antenna element modules in the array. The transmission lines couple signals applied to any of the ports to selected elements in all the antenna element modules of the array. This antenna, herein referred to as a COMPACT antenna, provides an effective element aperture which is coextensive with the array aperture.
Still another effective prior art antenna coupling network is described by Wheeler in U.S. Pat. No. 4,143,379, incorporated herein by reference and assigned to the same assignee as the present invention. According to the technique of Wheeler, cross coupling ports are employed to couple wave energy signals to modules which are contiguous to each module.
Yet, another technique is shown in U.S. Pat. No. 4,168,503 which describes an antenna array with a printed circuit lens in a coupling network. A radiated signal, received by each one of a plurality of spatially separated antennas forming a directive array, is coherently recovered by the lens. The lens comprises a plurality of vertically standing and circularly arranged printed circuit panels, each of which includes a conductor strip connected at one end to each antenna. A plurality of semi-elliptical circuit panels are affixed to the vertical panels at a predetermined angle. Metal strips plated on the semi-elliptical panels provide the desired time delay to the antenna signals. A combining strip couples the time delay strips and provides a combined output signal at one end of the semi-elliptical pattern. The angle at which the semi-elliptical boards are affixed to the vertical boards corrects for time delay distortion caused by the placement of the combining strip. This configuration cannot be implemented using printed circuit techniques on a single substrate layer.
U.S. Pat. No. 4,321,605 describes an array antenna system having at least a 2:1 ratio of antenna elements to input terminals interconnected via primary transmission lines. Secondary transmission lines are coupled to and intersecting a selected number of the primary transmission lines. Signals supplied to any of the input terminals are coupled primarily to the elements corresponding to the input terminal, and are also coupled to other selected elements.
In time reference scanning beam systems such as microwave landing systems (MLS), there may be a linearity requirement for the glide path guidance i.e., the difference between the actual and indicated angle must be within a limited range. There is also a requirement to minimize the field monitor distance for the glide path antenna. Particularly in MLS, this invention provides a non-thinned or fully filled array which may be used to achieve linearity and minimize the field monitor distance.